At present, crude oil and gas extraction, and geological or geothermal bores are realised by help of drilling rigs where rock is disintegrated by rotating drilling heads mounted at the end of assemblies of connected basic piping and rotated by driving units on land surface. Disintegrated rock is transported to land surface by help of special liquid circulating in the piping and in the drilled hole. There were efforts to put the driving units close to the drilling head and to bring energy from the land surface, but with transport of the crushed rock in classical manner—by help of highly viscous, quick-circulating liquid.
Primarily during the last decade, new methods of more effective rock disintegration and transport to land surface have been sought for.
In the latest study made at MIT (USA) “THE FUTURE OF GEOTHERMAL ENERGY”—IMPACT OF ENHANCED GEOTHERMAL SYSTEMS (EGS) ON THE UNITED STATES IN THE 21ST CENTURY 2006 the principal importance of resolving an economical method of making deep geothermal boreholes is pointed out. With current drilling technologies, the bore price grows exponentially with its depth. Thus, finding a boring technology allowing approximately linear growth of bore price and depth is an imperative challenge.
In his presentation, Jefferson Tester, a co-author of the above study, characterises the requirements related to a new, fast and ultra-deep boring technology as follows:                linear growth of the price of the bore with depth        neutral floating of the bore axis        the ability to make vertical or inclined boreholes more than 20 km deep        the ability to make large diameter boreholes—up to five times larger than on land surface        casing formed on site in the borehole.        
Above 20 innovative technologies of geological formation boring are known, with various maturity and verification levels.
Only the most promising ones, and those verified already, will be described within the state-of-the art.
Survey of Current Technologies
Technologies can also be evaluated according to properties such as specific energy needed to extract one cubic centimeter, maximum power applicable at borehole bottom, or maximum drilling rate achievable.
From the above viewpoints, the following methods are on the leading places: mechanical principles, underwater electro-spark discharge, and water jet cutting.
The extrapolation solutions which still lack the radical innovation properties necessary for deep geothermy include the following examples:                drilling by help of rotary casing (TESCO CASING DRILLING)—one set of piping is removed, but the principal negative features of mechanical boring remain unchanged;        composite coil piping with electric conductors for downhole drive (HALLIBURTON/STATOIL-ANACONDA)—the technology avoids the rotary boring pipe element used for mechanical energy transfer, only the function of crushed rock flush-out remains.        
A considerable progress towards a significant innovation is represented by U.S. Pat. No. 5,771,984, authored by Jefferson Tester et al.: “CONTINUOUS DRILLING OF VERTICAL BOREHOLES BY THERMAL PROCESSES: ROCK SPALLATION AND FUSION”, where energy is supplied to the drilling rig at borehole bottom by pressurised water for borehole flushing and for driving the turbine, and for generating electric energy for the drilling process by thermal spallation or melting of rock. This invention is the basis for the work carried out by Potter Drilling LLC company, whose technologies are in the prototype testing stage already.
Related technologies are described in v U.S. Pat. No. 5,107,936 “Rock Melting Excavation Process” in which the author Werner Foppe describes the process of rock melting along the borehole circumference, pressing the melt into the core and subsequent core disintegration. In U.S. Pat. No. 6,591,920 the same author describes rock melting and pressing thereof into the surrounding ground.
Plasma jet rock cutting is described in U.S. Pat. No. 3,788,703 authored by Thorpe; however, removal of crushed rock is not covered.
At Tel Aviv. University, Jerby et al. described rock spallation by local microwave overheating in Journal of Applied Physics 97 (2004). The technology is applicable to very small volumes so far.
Most patents refer to water jet rock cutting.
Different modification variants are described, e.g. utilisation of cavitation, turbulent processes, combination with mechanical processes, etc. For example, U.S. Pat. No. 5,291,957 describes the water jet process combined with turbulent and mechanical processes.
During the recent decade intense research has been made into utilisation of high energy laser beams for rock disintegration. Primarily conversion of military equipment is concerned.
Laser energy is used for the process of thermal spallation, melting, or evaporation of rock.
The patent by Japanese authors—Kobayashi et al.: U.S. Pat. No. 6,870,128 LASER BORING METHOD AND SYSTEM describes laser boring with the light beam carried from the ground to the borehole bottom via optical cable. The system evaporates rock, and thus high energy demand results.
In the paper LASER SPALLATION OF ROCKS FOR OIL WELL DRILLING, published in Proceedings of the 23rd International Congress on Applications of Lasers and Electro-Optics 2004, Zhiyue Xu et al. describe thermal spallation method which is more advantageous as to energy, but crushed rock is being removed by help of classical flushing.
The methods utilising electric discharge are based on long-term experience gained in other application areas. The method described in U.S. Pat. No. 5,425,570 by G. Wilkinson is based on combination of electric discharge and subsequent explosion of a small dose of explosive or induced aluthermic process.
U.S. Pat. No. 4,741,405 and U.S. Pat. No. 6,761,416 by W. Moeny describes the use of multiple electrodes with high voltage discharge in aquatic environment; crushed rock is removed by help of classical flushing.
A similar method is described in U.S. Pat. No. 6,935,702 by Okazaki et al.—“CRUSHING APPARATUS ELECTRODE AND CRUSHING APPARATUS”, with classical flushing used.
A. F. Usov describes utilisation of electric discharge for large diameter (above 1 m) drilling with several m/h speed, realised at the Kola Research Centre, Russian Academy of Sciences.
In the patent RU 2059436 C1, V. V. Maslov describes generation of high voltage pulses for material destruction.
In the paper “Pulsed Electric Breakdown and Destruction of Granite” published in Jpn. J. Appl. Phys. Vol. 38 (1999), 6502-6505, Hirotoshi et al. describe successful use of electric discharge on granite, a typical geothermal rock.
Utilisation of buoyancy in boring is not new; for example, in U.S. Pat. No. 4,422,801 “Buoyancy System for Large Scale Underwater Risers” Hale et al. describe undersea utilisation of buoyancy to lift heavy burdens, where effective manipulations are achieved by variable buoyancy of ballast vessels, although at high costs.
U.S. Pat. No. 5,286,462 by J. Olson describes the system of quick gas generation for fast discharge of ballast vessels to make use of buoyancy for load manipulation.
The problem of fast movement of an object in water—a key factor for transport efficiency—is handled for military purposes in U.S. Pat. No. 6,962,121 BOILING HEAT TRANSFER TORPEDO by R. Kuldinski, and in U.S. Pat. No. 6,684,801 SUPERCAVITATION VENTILATION CONTROL SYSTEM; here the artificial supercavitation method is described, with which objects of suitable shape can reach the velocity of even several hundreds of meters in water.
Apparatus for deep simulation at borehole bottom and the importance of pressure generation at borehole bottom by autonomous power system are described in U.S. Pat. No. 4,254,828 APPARATUS FOR PRODUCING FRACTURES AND GAPS IN GEOLOGICAL FORMATIONS FOR UTILIZING THE HEAT OF THE EARTH by Sowa et al. Similarly, U.S. Pat. No. 7,017,681 by Ivannikov et al. describes an autonomous simulation system utilising hydrodynamic effects at borehole bottom.
From the viewpoint of realisation of continuous casing production, the current state-of-the-art offers a suitable solution, because concrete mixtures with quick underwater solidification and high strength have been developed and introduced into practice, mostly for military purposes. Such concrete types have been developed for storage of dangerous waste as well.
Summary of State of Current Technologies
However, none of the above methods was successful in reaching substantial saving during boring, due to simultaneous effect of several factors:                transport of extracted material to the ground remained unsolved        supply of energy        considerable energy demand—the need to crush the entire borehole volume to small particles, or even (with laser technologies) to evaporate it.        
Effectiveness of the above technologies is also opposed by the presence of liquid (water, viscous transport liquid) in the borehole. To supply the energy, e.g. pressurized water supply, electric energy supply via a cable, composite flushing pipe, optical fibre cables supplying high-power laser energy were used. All of them assume a permanent, constantly extending connection of the borehole bottom with the ground. Similarly, crushed rock transport still depends upon extending transport medium piping.
An equally important part of the borehole is casing of its walls by subsequently inserted pipes which, moreover, are narrowing with borehole length, and thus cause overall throughput reduction and contribute to inadequate boring price increase with bore depth. Recently, expandable casing with uniform cross section along the whole borehole has been developed; this, however, provides a partial solution of exponential boring price only.
None of the boring technologies described so far brought an innovation which would bring along a substantial change in effectiveness of the entire process and of transport of crushed rock to the ground, and which would provide for ultra-deep boring (above 5 km) with approximately linear price dependence guaranteed. The status described above thus implies that a technology is needed which would avoid the cons of the current situation in relation to the following aspects:                Transport of energy downwards to the boring process.        Transport of crushed rock upwards so that direct continuous connection between the ground and the boring rig at borehole bottom would be abandoned in a manner independent upon actual borehole depth.        The casing process would be continuous, parallel with borehole formation.        Achieving energy savings in relation to rock disintegration and transport to the ground.        The possibility to cut rock into blocks and to transport them to the ground.        Functioning ability of the equipment even under high pressures and temperatures in boreholes (openings in rock) flooded with water.        